1) Technical Field
The present exemplary embodiments relate to a device for detecting an abnormality or failure of a gas sensor.
2) Description of the Related Art
Gas sensors have been employed in various fields. For example, a gas sensor is provided in an exhaust pipe of an internal combustion engine to detect a gas concentration such as a concentration of oxygen of an exhaust gas emitted from the internal combustion engine body so that the detection signal is used for the control of parts of the engine body.
In general, a gas sensor for internal combustion engines is recently made of an oxygen ionic conductive solid electrolyte material such as zirconia. For example, there has been known a gas sensor designed such that a chamber is formed to take oxygen in and out between the exterior of the gas sensor in which a gas to be measured exists and the interior of the gas sensor and the oxygen is introduced/ejected into/from the chamber through the use of a cell having a pair of electrodes on its solid electrolyte material (member). In this gas sensor, a voltage is applied between the pair of electrodes through signal lines connected thereto to move (pump) oxygen ions serving as a carrier in the interior of the solid electrolyte material to take oxygen in and out. Moreover, a limiting current corresponding to the oxygen concentration in the interior of the chamber is applied between the electrodes to detect a current flowing through the signal lines for detecting the oxygen concentration. In addition, there has been known a gas sensor in which a plurality of cells each having such a construction are provided to detect NOx, CO and HC additionally.
Meanwhile, in the recent years, in internal combustion engines and others, the presence or absence of various types of abnormalities are self-diagnosable in working conditions or the like. Japanese Patent Laid-Open No. HEI 11-6812 discloses an apparatus for making a decision on the occurrence of a trouble of a gas sensor. In particular, this apparatus is designed to make a decision on a current-carrying failure of a heater built in the gas sensor, and a decision is made as to the presence or absence of malfunction or abnormality such as disconnection or breakage by, when the heater is energized, making binary judgment on a voltage or current on the basis of its magnitude.
Accordingly, it is desirable that a disconnection between the cell electrodes and the signal lines or in the middle of the signal lines is detected through the use of the technique disclosed in the above-mentioned document to quickly make a decision as to a state of difficulty being encountered in achieving the normal detection of gas concentration. For example, it is considered that a voltage is applied between the electrodes of the cell to detect the presence or absence of a current.
However, the trend in the gas sensor is toward the size reduction and multi-cell construction, and the occurrence or non-occurrence of disconnection is not always detectable with accuracy on the basis of the current detection. This is due to the interference with a heater or cell integrated therewith.